In the field of injection molding using thermoplastic resin, it has heretofore been studied to perform expansion for the purposes of material saving, weight reduction, etc. As a method of conducting such expansion injection molding, a method is known which comprises use of a so-called “shear edge” mold designed so that a concave mold fits with a convex mold to form a cavity thereinside and the molds are caused to slide at their fitting portion to change the volume of the cavity; in the method, resin is injected into the cavity of the molding machine and then the volume of the cavity is enlarged. According to this method, it is possible to achieve an increased expansion ratio to exert an enhanced weight reduction effect.
In the above-mentioned expansion injection molding process, however, the external surface of an expansion molded article will have a great amount of defect in appearance, called swirl mark, which appears when expanding gas dissolved in molten resin breaks and the part is drawn. Therefore, such articles could not be used as practical products required having good appearance.
As a method for improving such swirl marks, Patent Document 1 discloses a technique where a thermoplastic resin containing an expansion agent is plasticated and then injected and charged into a cavity and subsequently the cavity volume is enlarged. Even in this method, however, it is difficult to retain the pressure of resin charged when the injection speed decreases at a flow front or at a section where the shape of a molded article changes greatly; and it was impossible to completely suppress the occurrence of swirl marks in the external surface of a molded article.
On the other hand, as another method for improving swirl marks, a so-called gas counter pressure method is known in which a resin containing an expansion agent is injected into a cavity in a mold, which has been pressurized with gas. However, as shown, for example, in Patent Document 2, the gas pressure of counter pressure generally requires a compression force of 1 MPa or more and airtightness of a mold is required. Therefore, the cost of molds was high. Moreover, in order to add such a high pressure to the inside of a mold, it takes time to increase the pressure of the gas in the cavity itself and therefore a cycle time itself is also long. This is a major factor of high product costs.
As a molding method solving this problem, Patent Document 3 proposes a technique in which airtight sealing is omitted by means of reduction in gas pressure. This method, however, is one conducting expansion of a level where shrinkage and warpage are prevented. It is slight expansion molding where a reduced amount of expansion agent is blended into resin and the expansion ratio is controlled to be about 1.0 to 1.2. For this reason, it cannot be used in expansion molding in which the cavity volume is enlarged to increase the expansion ratio. Moreover, the pressure of the gas in a cavity is successfully increased for a moment by such a method using no sealing. However, the increase continues only for a moment and it was impossible to keep the pressure during injection.
In conventional counter pressure molding, a time for supplying gas is required after forming a cavity between a fixed mold and a movable mold by moving the movable mold. Therefore, there are limits in speeding up of the molding process.
As conventional counter pressure molding, a method is known in which charging and discharging of counter pressure gas into and from a cavity through a gap of a parting portion in the periphery of the cavity is conducted in a non-shear edge type mold system where a fixed mold and a movable mold abut together. In such a mold, however, the gap of a parting portion is designed to be narrow in order to prevent leakage of molten resin. A large flow resistance is produced when gas passes through the parting portion when the gas is charged into and discharged from the cavity and, therefore, there are limits in speeding up of the charging and discharging of gas.
In particular, in order to increase the pressure in a cavity, it is necessary to supply compressed gas with a pressure not lower than the atmospheric pressure through a pipe after closing the mold. Therefore, the time for increasing the gas pressure in the cavity makes the molding cycle longer and a controlling device for controlling the suction and discharge of gas must be provided externally.
In injection molding, the molding cycle is one of the factors having a very important effect on cost. In conventional methods, a step of supplying gas into a cavity to increase the pressure after closing a mold is added. For this reason, the above-mentioned counter pressure method, which makes the molding cycle longer, is difficult to be adopted.
As a technique for improving the above-mentioned problems, Patent Document 4 proposes a technique in which a fixed mold and a movable mold having a shear edge structure are used and the pressure is increased simultaneously by utilizing mold closing.
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-120252
Patent Document 2: Japanese Examined Patent Application No. H1-22132
Patent Document 3: Japanese Patent Application Laid-Open No. H5-269778
Patent Document 4: Japanese Patent Application Laid-Open No. H11-277575